The problem of increasing output of neodymium lasers, particularly at specific wavelengths such as 1.44 .mu.m, is a difficult and much studied problem.
In the design of neodymium lasers having output in the wavelength range near 1.44 .mu.m (about 1.4-1.5 .mu.m, corresponding to radiation resulting from the transition .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.5/2, the actual wavelength of the output being variable depending on the host medium), several factors affect the power output. These include the length (L) and diameter (D) of the host crystal, the level of doping (N) of the host crystal with neodymium, the intensity of the excitation applied to the crystal and the reflectivity (R) of the mirrors defining the optical cavity.
As an example of the difficulties in increasing the power, arbitrarily increasing the intensity of the excitation applied to the crystal increases thermal lensing within the crystal, with a resulting degradation of the output beam. Further, while the gain of the laser is dependent on L (gain is proportional to e.sup.L), it is difficult to grow longer crystals, and in any event the expense of growing longer crystals (larger than 15 cm) is not worthwhile since radiation at 1.06 .mu.m is difficult to suppress with increasing L. In addition, the NdYAG laser is self-absorbing at 1.44 .mu.m, hence increasing L tends to increase the self-absorption.
Similarly, increasing the diameter D of the crystal does not in general yield increased power, since increased D results in thermal lensing and uneven pumping.
Neodymium lasers are usually operated with a neodymium level of 1N, where N (for Normal) is defined as a doping level of 1.1% neodymium by weight of the host crystal. This level is determined by the optimum level of luminescence of the output of a neodymium laser as the neodymium concentration varies. A graph showing the dependence on neodymium concentration of the luminescence at 1.06 .mu.m for a neodymium laser is shown in Laser Crystals, by Alex A. Kaminskii, Springer, at page 330. The luminescence peaks at the 1.1% concentration level, and decreases fairly rapidly below doping levels of about 0.7N to about half the maximum for a doping level of about 0.3N.
For a NdYAG laser with a given N, the reflectivity R of the output mirror is normally chosen according to the Rigrod model, which is a standard and well known model. In this model, the lasing threshold is determined as the level at which the lasing gain equals the combined losses due to absorption in the host crystal and transmission through the output mirror. Normal operation of the laser is then carried out at about five times the threshold. Since increasing N increases the absorption for output near 1.44 .mu.m, without sufficiently increasing the gain, an increase of N increases the lasing threshold. This, in combination with the limiting factors on crystal size and the applied excitation, has previously been considered to eliminate the neodymium crystal as a host medium for a laser producing output at 1.44 .mu.m.
As described in my U.S. Pat. No. 5,048,034, I have achieved satisfactory output from a neodymium laser at about 1.44 .mu.m by pulsing the excitation applied to the host crystal, and using discriminatory optics that discriminate against the 1.06 .mu.m radiation. This was a surprising result in that this line of output of the neodymium laser was considered too weak to provide useful power. This output was achieved using an output mirror having reflectivity of about 80% at 1.44 .mu.m and a host crystal with neodymium doping of 1N.
I have also discovered that operating the laser with a neodymium doping level between 0.3N and 0.7N, yields surprisingly higher output, particularly when the doping level is about 0.4N and the reflectivity of the output mirror is about 90%, rather than the 80% that is predicted by the Rigrod model.
In one aspect, therefore my invention provides a long wavelength neodymium laser, comprising:
a housing including a reflective hollow optical cavity;
a laser rod mounted in said optical cavity, said laser rod composed of a crystalline or glass host structure having neodymium doping;
a pump lamp mounted in said optical cavity adjacent to said laser rod, said pump lamp providing a source of light for transfer to said laser rod;
drive means including circuit means for driving said pump lamp to produce intermittent pulses of light from said pump lamp;
wavelength selective resonator means for providing maximum reflection at 1.4 to 1.5 .mu.m wavelength and minimum reflection at other wavelengths to allow laser oscillation in the range 1.4 to 1.5 .mu.m; and
the laser rod having a neodymium concentration level of between about 0.3 and 0.7N.
In a further aspect, the neodymium concentration is about 0.4N and the wavelength selective resonator means includes a mirror having reflectivity of about 90% at 1.44 .mu.m.